Turning plastic bar

I have to make several knobs from plastic , max diameter about 1/2", and put a 4-40 thread thru' the centre. the question is , What is the best shape of tool to use an will standard ME taps be OK. Thankyou.

Honestly, I think you should be okay with your normal turning equipment, i wouldn't say there's a best but i use carbide, you could try diamond coated drills for glass filled if you're worried about a decent finish. Keep the speed fast. 

Michael W

Edited By Michael Walters on 04/08/2016 20:25:37

Do you know what plastic it is?

Highish top rake usually works best so whatever you normally use for aluminium would be a good starting point.

Tapping will depend a lot on te specific plastic.

My attempts at turning plastic have all ended badly. Could be me getting it wrong (again) or perhaps I've been using unsuitable scraps. Are there plastics that don't turn well? The tool was sharp HSS at up to 1200rpm. Michael's comment about the need for speed has left me wondering if that was fast enough?

Thanks,

Dave

I find Acetal (Delrin) and HDPE are lovely to machine and tap using standard tooling. Finish on Delrin is very smooth.

The Noggin End Metals site has a plastics section and there are a few words of the properties in each section.

Hope that helps.

Alan

Thanks Alan - I'll order some Acetal!

Regards,

Dave

Speed is what you don't need. What you do need are sharp, high rake tooling, you can use high feedrates, and modest speeds. I normally use speeds appropriate to low carbon steel, but 3-5 times the feedrate. The problem with high surface speeds is that you end up melting the plastic; equals a gooey mess!

Andrew

I have not had problems turning Delrin but have found PTFE difficult. The first few cuts with a freshly ground tool are great producing a good finish. After that the finish becomes poor and eventually the PTFE tears. I have a feeling that the local heat at the close to the cutting edge of the tool produces very small amounts of HF. The amount HF produced being so small that it is not significant as a health risk, rapidly reacting with anything it can find, but enough to take the edge off the tool.

JA

Can't say I've seen that? I've always got a good finish on PTFE. The problem with PTFE is when it comes to holding tolerances. It seems to be all over the place. If you get it in tolerance, and then measure it the next day it might have changed by several thou. Creepy stuff! Glass filled PTFE is a different animal, it turns and mills beautifully and seems to hold tolerances without any issues.

Andrew

Try taking all the material off in one cut, small cuts can be ineffectual due to the plasticity. You honestly don't need special tools, your normal turning equipment should be more than up for the challenge. If you can remove the swarf quickly enough then your bar wont suffer ill effects, this melting mainly occurs in drilling when the swarf gets stuck on the outside of the lips and flutes of the drill. 1500 rpm should be sufficiently quick.

Michael W

Edited By Michael Walters on 05/08/2016 05:20:00

Delrin is very good to machine, the one I have problems with is polypropylene, it's either me or the tool but for most plastics the swarf coming off which is a continuous length just shoots out and lands on the floor, but when I turn polypropylene it builds up in front of the tool and wraps itself around the bar and I have to keep on switching off to remove it, it's impossible to machine up to a shoulder without having to clear it.

John

Edited By Sandgrounder on 05/08/2016 06:11:25

www.directplastics.co.uk will tell you all you need to know about plastics and their machining properties. Two good ones for your knobs would be Nylon 6 or Nylon 66. Both machine to a lovely finish and are hard wearing. If you have a SHARP hss tool with lots of top rake no problem, failing that you would need to use a carbide insert with a sharp geometry such as that specifically for aluminium and plastics.

I use directplastics and find them and their material to be of a good standard, their prices are good too. I recently made up some nylon and just used garden variety carbide insert tools and sharpened a blacksmiths drill by using the 4 facet technique, i then thin the web in the middle by tucking it into the radius' edge of the grinding wheel, oil slipstone to finish on the lips of the chisel point. 1500RPM (top speed for me) and drilled with fast feed, all in one go and retract quickly, beautiful finish on the bore and on the turning, like the boots of the calvary on horseguard's parade.

Michael W

Edited By Michael Walters on 05/08/2016 18:11:44

I used PTFE to make Graham Meek type spark plugs and found once I got a good finish I could get a plus/minus 0.002" tolerance. I did not let the PTFE rest after machining, the assembling and crimping of the plug body was done almost immediately. On a 1/4" to 5/16" diameter I don't think I exceeded a turning speed of 600 rpm.

JA